Rewrite expireRollbackForPackage (3/n)

Split the method into 2 parts to uphold thread invariants.

Note since the original expireRollbackForPackage do the expiration
synchronously, we have to block the caller thread until the task is done
on the handler thread.

This dispatch-and-wait pattern will be used in the following CLs.

Bug: 145335013
Test: atest CtsRollbackManagerHostTestCases
Test: atest RollbackTest
Test: atest StagedRollbackTest
Change-Id: Icf39d70c4f9af77b41598d341063e35bb5c900ac